1. Field of the Invention
The present invention relates to an oxide sintered body and a production method therefor, a target, and a transparent conductive film and a transparent conductive substrate obtained by using the same, and in more detail, the present invention relates to a target for sputtering or a tablet for ion plating, which enables to attain high rate film-formation and a nodule-less, an oxide sintered body suitable for obtaining the same and a production method therefor, and a transparent conductive film having low absorption of blue light and low specific resistance, obtained by using the same.
2. Description of the Prior Art
A transparent conductive film has high conductivity and high transmittance in a visible light region, therefore, has been utilized in an electrode or the like, for a solar cell or a liquid crystal display element, and other various light receiving elements, as well as a heat ray reflection film for an automotive window or construction use, an antistatic film, and a transparent heat generator for various anti-fogging for a refrigerator showcase and the like.
As a well known practical transparent conductive film, there has been included a thin film of tin oxide (SnO2)-type, zinc oxide (ZnO)-type, indium oxide (In2O3)-type. As the tin oxide-type, one containing antimony as a dopant (ATO), or one containing fluorine as a dopant (FTO) has been utilized, and as the zinc oxide-type, one containing aluminum as a dopant (AZO), or one containing gallium as a dopant (GZO) has been utilized. However, the transparent conductive film most widely used industrially is the indium oxide-type. Among them, indium oxide containing tin as a dopant is called an ITO (Indium-Tin-Oxide) film, and has been utilized widely, because, in particular, a film with low resistance can be obtained easily.
The transparent conductive film with low resistance is suitably used widely in a surface element or a touch panel or the like, such as for a solar cell, a liquid crystal, an organic electroluminescence and an inorganic electroluminescence. As a production method for these transparent conductive films, a sputtering method or an ion plating method has been used often. In particular, a sputtering method is an effective method in film-formation of a material with low vapor pressure, or when control of precise film thickness is required, and because of very simple and easy operation thereof, it has been widely used industrially.
In a sputtering method, a target for sputtering is used as a raw material of a thin film. The target is a solid containing a metal element constituting the thin film to be formed, and a sintered body such as a metal, a metal oxide, a metal nitride, a metal carbide, or, in certain cases, a single crystal is used. In this method, in general, after making high vacuum once with a vacuuming apparatus, rare gas such as argon is introduced, and under a gas pressure of equal to or lower than about 10 Pa, a substrate is set as an anode and a target is set as a cathode to generate glow discharge between them and generate argon plasma, and argon cations in the plasma are collided with the target of the cathode, and particles of the target component flicked thereby are deposited on the substrate to form a film.
A sputtering method is classified by a generation method of argon plasma, and a method using high frequency plasma is called a high frequency sputtering method, and a method using direct-current plasma is called a direct-current sputtering method.
In general, a direct-current sputtering method has been utilized industrially in a wide range, because it provides higher film-formation rate and lower cost of power source facility and simpler film-formation operation, as compared with the high frequency sputtering method. However, the direct-current sputtering method has a disadvantage of requiring use of a conductive target, as compared with the high frequency sputtering method, which can provide film-formation even by using an insulating target.
Film-formation rate of a sputtering has close relation to chemical bond of a target substance. Because a sputtering is a phenomenon that argon cations having a kinetic energy are collided to the target surface, and a substance of a target surface is flicked by receiving energy, the weaker inter-ionic bond or inter-atomic bond of the target substance increases the more probability of jumping out by sputtering.
In film-formation of a transparent conductive film of an oxide such as ITO by using a sputtering method, there are a method for film-formation of an oxide film by a reactive sputtering method in mixed gas of argon and oxygen, by using an alloy target (an In—Sn alloy in the case of the ITO film) of metals constituting the film, and a method for film-formation of an oxide film by a reactive sputtering method for performing a sputtering in mixed gas of argon and oxygen, by using an oxide sintered body target (an In—Sn—O sintered body in the case of the ITO film) of elements constituting the film.
Among these, in a method for using the alloy target, relatively high amount of oxygen gas is supplied during sputtering, however, because dependence of film-formation rate or film characteristics (specific resistance, transmittance) on amount of oxygen gas to be introduced during film-formation is extremely high, it is difficult to produce stably a transparent conductive film having a constant film thickness or characteristics.
In a method for using the oxide target, a part of oxygen to be supplied to a film is supplied from the target by sputtering, and residual deficient oxygen amount is supplied as oxygen gas. Therefore, dependence of film-formation rate or film characteristics (specific resistance, transmittance) on amount of oxygen gas to be introduced during film-formation is lower as compared with the case where the alloy target is used, and a transparent conductive film having a constant film thickness or characteristics can be produced more stably, and for this reason, a method for using the oxide target has been adopted industrially.
Under such background, in the case where mass production of film-formation of a transparent conductive film is performed by a sputtering method, a direct-current sputtering method used an oxide target is adopted in most cases. In consideration of productivity or production cost here, characteristics of the oxide target in direct-current sputtering becomes important. That is, the oxide target providing higher film-formation rate under the same charging power is useful. Further, because charging of the higher direct-current power increases film-formation rate the more, such an oxide target becomes useful industrially that is capable of providing film-formation stably, without generation of target crack or abnormal discharge such as arcing caused by nodule generation, even when high direct-current power is charged.
The nodule here indicates a black precipitate (a protrusion substance) generating at an erosion part of the target surface, excluding only a trace part at the deepest part of the erosion, when the target is being sputtered. In general, the nodule is said not to be a deposition of a foreign flying substance or a reaction product at the surface but a digging-residue left by sputtering. The nodule causes abnormal discharge such as arcing, therefore by reduction of the nodule, arcing is suppressed (refer to Non-Patent Document 1). Therefore, an oxide target which does not generate the nodule, that is, digging-residue left by sputtering, is suitable.
On the other hand, an ion plating method is a method where a metal or a metal oxide is evaporated by a resistance heating or electron beam heating, and further the evaporated substance is deposited onto a substrate after activation with plasma along with reactive gas (oxygen). Also as for a target for ion plating (which may be called a tablet or a pellet) to be used for formation of a transparent conductive film, similarly to a sputtering target, use of an oxide tablet is capable of producing more stably a transparent conductive film having constant film thickness and constant characteristics. The oxide tablet is required to evaporate uniformly, and it is preferable that a substance having stable chemical bond and difficult to be evaporated is not present together with a substance which is present as a main phase and easily evaporated.
As described hitherto, although ITO, which is formed by a direct-current sputtering method or an ion plating method, has been used industrially in a wide range, in a LED (Light Emitting Diode) or an organic EL (Electro Luminescence) whose progress has been significant in recent years, the cases have been emerged where characteristics not obtained by ITO is required. As one example, in a blue LED, it has been required a transparent conductive film with high transmittance of blue light of a wavelength of around 400 nm, to enhance light extraction efficiency. Light absorption of ITO in a visible region tends to increase more in the shorter wavelength region, in the order of red light, green light and blue light. Therefore, use of ITO as a transparent electrode of the blue LED results in loss by light absorption.
In order to avoid such a problem, in recent years, as an electrode of various light emission devices, there has been proposed a transparent conductive film composed of an oxide film containing indium and gallium, as an ITO-alternative transparent conductive film having low absorption of visible light, in particular, blue light. However, because of insufficient development of an oxide sintered body to be used as a target or a tablet, mass production of transparent conductive film having a good quality is not possible and thus practical applications have not yet been attained until now.
As for the transparent conductive film with low absorption of blue light, there has been proposed a transparent conductive material containing a gallium-indium oxide (GaInO3) doped with small quantity of different-valent dopant such as a tetra-valent atom (for example, refer to Patent Document 1). In this Document, there has been described that a crystal film of said oxide exhibits excellent transparency and a low refractive index of about 1.6, which improves refractive index matching with a glass substrate, as well as enables to attain electric conductivity in the same degree as that of a wide forbidden band semiconductor to be used presently. And, there have been described that in a green or blue region, this film exhibits transmittance superior to an indium-tin oxide, and uniform and slight absorption over a visible spectra, and also a pellet of a raw material to be used in film-formation of said oxide, is a material with a single phase having a GaInO3-type structure such as one represented as GaIn1−xMxO3.
However, in the case where the material with a single phase having the GaInO3-type structure is used as a target for a sputtering or a pellet for vapor deposition, sputtering yield is low and film-formation rate decreases extremely to about ½ of ITO, because said material with a single phase has a stable chemical bond, which is specific to a composite oxide, and thus it is disadvantageous industrially. In addition, there has been no study on a composition, a constitution or the like of an oxide sintered body, which makes possible suppression of nodule generation, which causes the above arcing, when condition to increase film-formation rate by increase in sputtering voltage or the like, is selected to overcome low sputtering yield. That is, as for the oxide sintered body as a raw material of the above transparent conductive film, there has been no consideration as far as an industrially practical aspect.
In addition, as a transparent conductive film and an oxide sintered body of the same oxide type as in the above Patent Document 1, there has been proposed a transparent conductive film having far higher conductivity than that of GaInO3 or In2O3, that is, lower specific resistance and excellent optical characteristics, in a composition range different from conventionally known GaInO3, and containing Ga content of from 15 to 49% by atom represented by Ga/(Ga+In), in a pseudo two-dimensional system represented by Ga2O3—In2O3 (for example, refer to Patent Document 2).
However, there is not so much detailed description on preparation condition of the oxide sintered body of a raw material for obtaining of the above transparent conductive film, and as for the constitution of the transparent conductive film, it is shown as an amorphous state or a microcrystalline state composed of a mixed phase such as GaInO3, GaInO3 and In2O3, GaInO3 and Ga2O3, however, there is no description on the oxide sintered body of a raw material. Therefore, similarly as described above, as for the constitution of the oxide sintered body of a raw material, there has not been found any approach on an optimal state from an industrial or practical point of view, such as film-formation rate or suppression of a nodule.
Further, as a transparent conductive film of the same oxide type as in the above Patent Document 1, there has been proposed a transparent conductive film characterized by being composed of In2O3 with a Ga content of from 1 to 10% by atom (which may be referred to as an IGO film), and having lower specific resistance and higher transmittance as compared with a conventional ITO film (a transparent conductive film composed of In2O3 added with Sn), which is formed by a film-formation method such as a sputtering method, as a transparent conductive film which enables to correspond to, in particular, a large sized, colorized and highly accurate liquid crystal display (LCD) (for example, refer to Patent Document 3). There has been described that the IGO film having the above constitution has lower specific resistance, such as a specific resistance of equal to or lower than 1×10−3 Ω·cm, even for a film formed at 100° C., as well as higher transmittance such as equal to or higher than 85% in a visible region, as compared with a conventional ITO film, therefore, it can be suitably used, in particular, as a transparent electrode of the LCD, and thus can attain high functionality and quality enhancement such as large sized, colorized and highly accurate tendency of LCD in the future. However, as a target to obtain the above transparent conductive film, there has been proposed only a composite target arranged with a Ga chip on an In2O3 target, or an In2O3 target containing predetermined quantity of Ga, and there has been no study on a constitution or the like of the oxide sintered body, thus it is not a approach on an optimal state from an industrial or practical point of view, such as film-formation rate or suppression of a nodule.
Under such circumstances, the present applicant has proposed a transparent conductive film of the same oxide type as in the above Patent Document, a sintered body target for production of a transparent conductive film, a transparent conductive substrate and a display device using the same (for example, refer to Patent Document 4). In this Document, there has been described, as for a structure of the oxide sintered body, that it is composed of Ga, In and O, and contains the above Ga from 35% by atom to 45% by atom, relative to total metal atoms, mainly composed of a GaInO3 phase with a β-Ga2O3-type structure and an In2O3 phase with a bixbyite-type structure, and X-ray diffraction peak intensity ratio to a β-GaInO3 phase (111) of an In2O3 phase (400) is from 50% to 110%, and density is equal to or higher than 5.8 g/cm3. In addition, as for conductivity of the oxide sintered body, specific resistance is equal to or lower than 4.0×10−2Ω·cm, and in the case where it is over this level, film-formation rate decreases and productivity decreases, even when DC magnetron sputtering is possible.
In addition, as for a composition and a constitution structure of the above oxide sintered body, it is only considered influence on specific resistance or light transmittance of a transparent conductive film formed from this as a raw material, and the Ga content of below 35% by atom results in decrease in light transmittance in a shorter wavelength region of visible light, and the Ga content of over 45% by atom decreases conductivity. The specific resistance is required to be from 2×10−3Ω·cm to 8.0×10−3Ω·cm, and although a region of the value below 1.2×10−3 Ω·cm is preferable, it requires for the composition of said transparent conductive film to have the Ga content of below 35% by atom. Accordingly, the optimal composition was selected in view of conductivity and optical characteristics of the obtained film, and there has not been studied on influence or the like of a composition and a constitution of the oxide sintered body on nodule generation, as a study item.
Under such circumstances, appearance of an oxide sintered body having indium and gallium has been desired, which has solved a practical problem such as high film-formation rate or nodule suppression that is important in mass production of a transparent conductive film containing indium and gallium, having low absorption of blue light and low resistance.    [Patent Document 1] JP-A-7-182924    [Patent Document 2] JP-A-9-259640    [Patent Document 3] JP-A-9-50711    [Patent Document 4] JP-A-2005-347215    [Non-Patent Document 1] “Technology of a transparent conductive film (the second Revised version)”, Ohmsha, Ltd., published on Dec. 20, 2006, p. 238 to 239